Wet-process phosphoric acid is produced by the acidulation of phosphate rock. This acidulation is generally accomplished by the treatment of phosphate rock with an acid, such as sulfuric or nitric acid. The acidulation process produces a phosphoric acid-calcium sulfate slurry plus other salts which require solids-liquids separation by filtration. The general equation for the process when sulfuric acid is utilized is: EQU 2Ca.sub.5 F(PO.sub.4).sub.3 +10H.sub.2 SO.sub.4 +(10.times.) H.sub.2 O.fwdarw.6H.sub.3 PO.sub.4 +10CaSO.sub.4.xH.sub.2 O+2HF
During the digestion of the ore with sulfuric acid, by-product calcium sulfate is formed. When the digestion takes place at about 80.degree. C. or below and the solids-free phosphoric acid is below 30% P.sub.2 O.sub.5 the by-product is calcium sulfate dihydrate or gypsum. This by-product is normally removed by filtration. Water can be evaporated from the resulting phosphoric acid product to increase the acid concentration. Typically, the digestion process produces a product prior to filtration that assays from 26% to 30% P.sub.2 O.sub.5. The evaporation step, if utilized, increases the P.sub.2 O.sub.5 concentration to 54% for merchant grade phosphoric acid or to above 70% for super phosphoric acid.
Normally, the production rate of the entire plant is dependent upon the filtration rate because in the past it has been the variable least subject to control. Impurities contained in the ore can markedly affect the filtration rate, as well as other operations of the process. Organic impurities adversely affect the wet process phosphoric acid manufacture for various reasons, which include the formation of stable froths in the digestor which impede the acidulation process. The organic impurities may also affect the size and shape of crystals of calcium sulfate which are formed and thus affect filtration rates, and the organic impurities may also lead to organic residues which coat the gypsum during filtration and thereby substantially reduce the filtration rate.
In order to remove organic impurities, phosphate rock has been calcined. While calcining of phosphate rock reduces the organic impurities, it may adversely affect the rock's reaction rate during the acidulation process by decreasing the reactivity or rate of reaction of the ore.
Impurities in phosphate rock can also be the source of corrosion. Excessive corrosion of plant equipment is undesirable in view of operating, fabrication, and maintenance costs. In copending application Ser. No. 338868, filed Jan. 11, 1982 and entitled "Multiple Stage Thermal Benficiation Process for Phosphate Ores" a multiple stage calcination process is disclosed which reduces not only the organic content but also the corrosive character of calcined ores. U.S. Pat. No. 4,277,454 discloses a method for controlling excessive corrosion in phosphoric acid circuits by oxidation of reduced ionic species in the acid within the digestion circuit to an EMF value above about 190 millivolts. While control of oxidation potential has been directed towards the prevention of corrosion, no effect on the filtration rate has been reported. In studying the effect of multiple stage calcination on the oxidation state of product phosphoric acid, an improvement in filtration rates have been observed for the more highly oxidized acids.
The present invention provides a method to increase the separation rate or filterability of the wet process phosphoric acid and by-product gypsum or calcium sulfate dihydrate. In the wet process, the degree of hydration of the calcium sulfate formed during the reaction or in any subsequent process steps is mainly controlled by the temperature, P.sub.2 O.sub.5 concentration, and the level of dissolved impurities. When the process is carried out at or below 80.degree. C. (176.degree. F.) and the solids free phosphoric acid is below 30% P.sub.2 O.sub.5, the degree of hydration of calcium sulfate is mainly as the dihydrate or gypsum.
A major factor in monitoring the economic and technical success of the process is the ease with which phosphoric acid can be separated from the by-product gypsum. Any procedure that enhances the rate of liquid-solid separation (usually filtration) has potential monetary value in that it permits increased production without new or significant capital investment. The present invention provides a method for increasing the filterability of wet process phosphoric acid by control of the total iron ion concentration in the acid and the ratio of ferric ion to ferrous ion in the acid. This can be achieved by adding a source of ferric ion, or oxidizing the iron naturally occurring in the ore, during digestion of the phosphate rock or by controlling the calcining temperature in the final stage of a multistage calciner or a combination of these.